This invention relates to high performance amplifier systems, and has particular application to systems for amplifying slowly changing low-level signals. Such amplifiers are in wide use, for example in measurement instruments having transducers which produce low-level voltages indicative of a condition being monitored.
Many problems must be overcome in building a high performance, low-level amplifier system. These include minimizing offset voltage (the differential voltage which must be applied at the input terminals of the amplifier to produce a zero volt output), minimizing noise (short term variations in the offset voltage), and minimizing drift (long term variations in the offset voltage with time and temperature).
Presently known high performance amplifier systems generally include an operational amplifier and a first (input) gain stage for amplifying information signals applied to the input of the amplifier system and applying them, in amplified form, to the input terminals of the operational amplifier. The first stage is typically, but not necessarily, a differential amplifier. Because the signals received by the first stage are very weak, and because the first stage has a substantial gain, the absolute offset, noise and drift of the input stage are far more significant for overall amplifier system performance than are these characteristics of the operational amplifier stage.
One way to reduce offset, noise and drift is by using closely matched (sometimes laser trimmed) transistors in the input stage of the system. An example of this approach is described in an article by Diamond and Siefert, Electronics, June 21, 1971, pages 76-80. This approach however, does not improve performance sufficiently for many applications. A particular problem, even with this approach, is that the noise component is not uniform with frequency. Instead, the noise contribution for a given bandwidth tends to rise below a corner frequency. This noise is commonly called "1/f noise" since it increases as frequency decreases. Since most low level amplifiers are used to amplify dc or slowly varying low-level signals, 1/f noise can become the dominating error term in a low-level amplifier.
To achieve adequate performance for low signal level applications, far more complex systems are presently required. A first such system is the so-called "chopper stabilized amplifier" system. Chopper stabilized amplifier systems are described for example, in Burr Brown Research Corporations' "Operational Amplifiers: Design and Applications" (1971) at pages 150-160, and a modern version is described in Electronic Design, Aug. 2, 1976, page 78. Briefly, a chopper stabilized amplifier system chops the dc input signal (e.g., by periodically switching between two signals) to generate an ac signal. The ac signal is coupled to a first amplifier, amplified, and then demodulated to a dc signal which is filtered and further amplified by a second amplifier. Because of the ac coupling, there is no offset voltage from the input stage. A chopper stabilized amplifier system is also particularly effective in reducing long-term drift. Such a system, however, suffers from drawbacks which go beyond its complexity and expense. Switching spikes generally introduce small offsets. Furthermore, the system's requirement for at least one large capacitor at the output side of the ac coupled first stage makes the circuit difficult or impossible to fabricate as a single semiconductor integrated circuit.
A second high performance amplifier system, sometimes also referred to as a chopper amplifier system, periodically shorts the input leads and amplifies the resulting offset signal. The offset signal is stored by a sample and hold circuit for use as a correction factor during the period that the input leads are not shorted. This second type of chopper circuit has been integrated on a semiconductor chip, but its performance is generally not as good as the first type. This second type of chopper circuit is described, for example, in Aumiaux, U.S. Pat. No. 3,748,587 (1973) and Johnson, U.S. Pat. No. 3,681,703 (1972).
Both types of chopper circuits are dependent on the switching circuit for their operation. If the clock stops, the amplifier system locks up. Further, both are dependent on capacitors for their operation and thus have a substantial recovery period if they are overloaded.